Business editor turns key (4)
I believe the answer is:
code
'key' is the definition.
(thesaurus)
'business editor turns' is the wordplay.
'business' becomes 'co' (short for company).
'editor' becomes 'ed' (abbreviation).
'turns' says the letters should be written in reverse.
'ed' reversed gives 'de'.
'co'+'de'='CODE'
(Other definitions for code that I've seen before include "System of ethics - cipher" , "Body of laws" , "(Computer) programming instructions" , "System used to disguise or represent words, like Morse" , "regulations" .)